Salt bath system and method for treating metals



United States Patent ice SALT BATH SYSTEM AND METHOD FOR TREATING METALSHarry R. Spence, Huntly, Va., assignor to Diamond Alkali Company,Cleveland, Ohio, a corporation of Delaware No Drawing. Application July25, 1952, Serial No. 300,985

10 Claims. (Cl. 1343) This invention relates to a new alkaline salt bathcomposition and to processes for conditioning and removing oxides on thesurfaces of ferrous and non-ferrous metals and alloys.

It is known to follow the alkaline bath treatment of a metal to bedescaled or cleaned with a light pickling treatment in a dilute aqueoussolution of an inorganic acid to complete the removal of oxide which hasbeen loosened and rendered easily removable by previous alkaline bathtreatment. The process of this invention is advantageous in that itreduces the time of treatment in the alkaline bath heretofore required,does not result in attack on the metals themselves as distinguished fromthe oxide thereon, reduces the rigorousness of the subsequent acidtreatment and thus saves acid, and, moreover, is useful when applied tometals known as stainless steel, such as chromium-iron alloys,nickel-iron alloys, and other chromium-nickel-iron alloys which re sistdescaling treatment by prior art alkaline salts in a fused state.

It has heretofore been proposed to employ alkaline salt baths in theremoval of oxides from steel surfaces. One prior practice of the generalnature involved employs a bath comprising one or more fused causticalkalies containing a minor proportion of an oxidizing agent, such asalkali nitrate. Experience has taught, however, that in employing such acomposition, excessive removal of metal, as well as oxides, isencountered, and uneconomical corrosive destruction of containers usedto hold the bath is encountered. Additional disabilities which accompanythe practice of such prior art processes, especially those where thenitrate is present in an amount of the order of twenty odd percent, andparticularly those wherein the remainder of the bath comprises an equalmixture of sodium and potassium hydroxides, is that in the treatment ofstainless and high chromiumcontent steel, there is commonly apreferential action upon the oxides in favor of the chromium oxide,tending to leave undue quantities of the remaining oxide unaffected andthus not subject to complete removal by subsequent acid picklingtreatment.

In accordance with the present invention, I have discovered that theoxides of all such metals can be removed or at least so conditioned bytheir immersion in my new fused salt bath that the various diflicultiesof the foregoing and other prior practices are overcome. In my newcomposition, I preferably employ an anhydrous alkaline salt which ispreponderantly 5651515 hydroxide. Suc lisodiiinihyd roxide may bepurchased in the form of commercial caustic soda which generally mayhave as much as about llblyleizhtnisodiumchlorige therein as a normalimpurity. My new salt includes between about 0.25% and not more than 10%by weight, and preferably about 5%, of potassium hydroxide (KOH) whichmay be of commercial purity. The remainder of the salt comprises acompound adapted to function as an oxidizing agent in the environment ofthe bath. Suit- 2,738,293 Patented Mar. 13, 1956 able agents includealkali oga lkaline earth dichromates, nitrates, peroxides, manganates,permanganates or chlo ra'te'sj in general, sodium salts are moresuitable and sodium nitrate is preferred. The oxidizing agent may alsobe of commercial purity. Further, it will be noted that the potassiumhydroxide ingredient is a minor percentage of the new composition. Theoxygen-containing ingredient, on the other hand, may vary in weight froma weight somewhat greater than the weight of the potassium hydroxide toa weight about equal to the weight of the sodium hydroxide.

In the application of my process, the salt is employed in a non-aqueousfused condition in an operative range of between about 650 F. and about1250 F., optimum results being obtained between about 800 F. and about950 F.

In the process of this invention as applied to stainless steels,nickel-chrome steels and straight chrome steels, oxides generally in theform of a scale which may be quite adherent before treatment arenormally removed. In the cases where complete removal is not effected inthe alkaline bath, the scale is changed so that a fine powderytype ofsurface is initially produced. No attack upon the metals themselves isobserved in either case. Another material advantage is that the time oftreatment is sub stantially reduced to as much as one-half the timerequired by the nearest comparable prior art procedures. Such an advanceis of enormous moment in the industry.

In those cases in which the oxides have been conditioned by the newprocess rather than completely removed, an acid dip of short duration ina dilute aqueous solution of an inorganic non-oxidizing acid, such assulfuric acid (H2504) or hydrochloric acid (HCl), will remove all traceof the conditioned oxides, leaving a clean and bright surface on themetals treated. In addition to providing for the removal of adherentscale, the present invention also incidentally frees the metal surfacesof oils and greases and removes all rust and impurities that may bethereon. Prior to the light acid pickling dip, it is desirable to rinseor wash the metals with water to avoid bringing the alkaline materialinto the acid-dip tank. The shorter period of time such metals need bein contact with the dilute inorganic acid is a highly advantageousaspect of this invention as any tendency of the dilute acid to attackthe metals themselves is correspondingly diminished. In some cases itmay be desirable to prolong the period of the acid dip to as much asfive minutes which, however, will not change the benefits achievable bymy new process. As will be well-understood by those in the art, thecomposition of the dilute acid may contain from 3% to 15% ofconcentrated acid by volume.

By way of example and not by way of limitation of the advantages of thenew process, the following specific examples are offered:

Example I A piece of hot rolled, annealed stainless steel having about18% nickel content, about 8% chromium content, with the balancesubstantially iron is immersed for about five minutes in a bathcontaining about 70% sodium hydroxide, about 28% sodium nitrate, andabout 2% potassium hydroxide, allsuch percentages being by weight. Thetemperature of the bath is about 900 F. Upon removal of the piece fromthe bath, it is washed with water and then dipped for about one minutein a tank of 3% sulphuric acid. Upon removal of the piece of stainlesssteel from the acid, it is found to have a clean, bright metal surfacewithout any trace thereon of the oxides that were present at the time itis immersed in the salt bath. In general, the times of immersion both inthe salt bath and in any subsequent dilute acid bath are not critical,except as otherwise taught herein, and may be varied to suit operationalrequirements.

Example II A piece of low nickel-content steel, the scale on which isgenerally very difiicult to remove, is treated in accordance withExample I. Scale and other surface impurities are completely removed.

Example III A piece of severely surface-oxidized stainless steel istreated in a bath of composition 92% by weight of sodium hydroxide, 5%of sodium nitrate, and 3% of potassium hydroxide. Water wash, followedby one minute 5% sulfuric acid treatment, removes all oxides.

Example IV A sample of steel plate, as in Example I, is immersed in amolten bath consisting essentially on a Weight basis of about 75% sodiumhydroxide, 22% sodium nitrate, and 3% potassium hydroxide. After oneminute treatment at 900 F., the sample is removed, water washed, andacid dipped in HQ for 3 minutes. The sample emerges bright andcompletely cleaned. A procedure similar in all respects but omitting the3% potassium hydroxide results in the removal of oxide to an extent ofabout only 50%.

Example V A. A hot rolled annealed plate of 18-8 stainless steel havinga heavy layer of oxide is immersed for five minutes in a fused bathcontaining 50% caustic soda, 45% sodium nitrate, and 5% by weight ofcaustic potash at 1000 F. When removed and quenchced in water, the oxideis found to be changed to a sponge-like material and is removedcompletely in a one minute dip in 5% sulfuric acid, leaving a clean,bright surface as though polished, without attack on the base metal.

B. A piece of the same plate as that in Example V A. is treated in abath of 50% caustic soda and 50% sodium nitrate for 5 minutes at 1000"F., all other conditions being duplicated. Only about 50% of the oxideis found to be changed. This plate requires 7 minutes in sulfuric acidto remove the sponge-like material. Upon inspection, there was stillevidence of the original oxide that was not afiected by the fused bathor the acid.

Example VI A piece of the same plate as that in Example V is treated inthe same way as in the first section of Example V, at temperatures of940 F. and 775 F. The results are substantially the same except at 775E, where the piece is noticeably less bright and shiny after acidtreatment.

Example VII A piece of the same plate as in Example V is treated for 10minutes in a fused bath of 50% caustic soda, 45% of sodium nitrate, and5% of caustic potash by weight at 875 F. The result is comparable tothat of the first section of Example V. Washing in water and one minute5% sulfuric acid dip leaves the surface clean and bright.

Example VIII A piece of type 430 straight chrome hot rolled and annealedplate (14-48% Cr, 1% Si, 1% Mn, 0.12% C, remainder substantially Fe)with a heavy thick oxide coating is treated in a fused bath of 50%caustic soda, 45% sodium nitrate, and 5% caustic potash for 5 minutes at850 F. When removed, the surface oxides appear to be changed to a softspongy material having a reddish' brown color. When washed in water andgiven a one minute dip in warm weak sulfuric acid, the spongy materialis removed, leaving a uniform clean white metal surface.

Example 1X A piece of the same plate as in Example VIII is treated in afused bath of caustic soda and 50% sodium nitrate by weight without anycaustic potash for 8 minutes at 850 F. When removed, the surface is adull, glazed, black color and does not have the spongy material. A waterwash and dip of one minute in weak warm acid remove only a small part ofthe black glaze.

Example X A piece of nickel steel with a heavy thick layer of blackoxide is treated as in Example V with equally satisfactory results.

Similarly satisfactory results are obtained with many other alloysincluding pure titanium and titanium-nickel chrome stainless steel andheat resisting alloys.

The theoretical basis for this invention is not completely apparent. Itappears that the combination of sodium hydroxide with 10% or less ofpotassium hydroxide has an action which is different from the action ofsodium hydroxide alone and less injuriously corrosive both to work andcontainers of the bath than baths comprising a major amount of potassiumhydroxide with caustic soda and an oxidizing agent.

While there have been described in detail certain forms of the inventionand embodiments of its practice, the invention is not to be understoodas being limited to the detailed disclosure as it is realized thatchanges within the scope of the invention are possible, and it isfurther intended that each step in the following claims shall refer toall equivalent steps for accomplishing the same result in substantiallythe same or equivalent manner, it being intended to cover this inventionbroadly in whatever form its principle may be utilized.

What is claimed is:

1. A composite salt for a metal cleaning salt bath process, consistingessentially of, on a weight basis, sodium hydroxide and sodium nitratein approximately equal parts and potassium hydroxide in an amount not inexcess of about 10% of the total weight of said salt.

2. A composite salt for a salt bath process, consisting essentially of,on a weight basis, potassium hydroxide from about 0.25% to about 5%,sodium nitrate in a quantity at least one and one-half times the weightof said potassium hydroxide but not in excess of the weight of sodiumhydroxide, and the balance substantially sodium hydroxide.

3. In a process for removing oxides from metals, the steps comprisingimmersing such metals in a bath of fused salt consisting essentially ofsodium hydroxide as the preponderant ingredient, potassium hydroxide ina quantity within the range from about 0.25% to about 10% by weight ofthe total dry weight of said salt, and an oxidizing agent selected fromthe group consisting of peroxide and oxygen yielding salts of the alkaliand alkaline earth metals, said agent being stable in the presence ofstrong alkali at a temperature between from about 650 F. to 1050 F. fora period of time sufiicient to change the oxides on said metals into aporous readily removable condition, said oxidizing agent being presentin a quantity of at least one and one-half times the weight of saidpotassium hydroxide but not in excess of the weight of sodium hydroxide.

4. In a process for removing oxides from metals, the steps comprisingimmersing such metals in a bath of fused salt consisting essentially ofapproximately equal parts of sodium hydroxide and sodium nitrate, andpotassium hydroxide in a quantity between about 0.25% and about 5% byweight of the total weight of said salt for a period of time sufficientto change the oxides on said metals into a porous and at least looselyadherent readily removable condition.

5. In a process for removing oxides from metals, the steps comprisingimmersing such metals in a bath of fused salt consisting essentially ofsodium hydroxide as the preponderant ingredient, potassium hydroxide inan amount within the range from about 0.25% to about 5% by weight of thetotal weight of said salt and an oxidizing agent selected from the groupconsisting of peroxide and oxygen yielding salts of the alkali andalkaline earth metals, said agent being stable in the presence of moltencaustic alkali in a quantity at least one and one-half times the weightof said potassium hydroxide but not in excess of the weight of saidsodium hydroxide for a period of time sufficient to change the oxides onsaid metal into a porous readily removable condition.

6. In a process for removing oxides from metals, the steps comprisingimmersing such metals in a bath of fused salt consisting essentially ofsodium hydroxide as the preponderant ingredient, potassium hydroxide inan amount within the range from about 0.25% to about 5% by weight of thetotal weight of said salt and the balance sodium nitrate in an amountgreater than the amount of potassium hydroxide but not greater than theweight of sodium hydroxide at a temperature between from about 650 F. to1050 F. for a period of time sufficient to change the oxides on saidmetals into a porous readily removable condition.

7. In a process for removing oxides from metals, the steps comprisingimmersing such metals in a bath of fused salt consisting essentially ofapproximately equal parts of sodium hydroxide and sodium nitrate, andpotassium hydroxide in a quantity between about 0.25% and about 5% byweight of the total weight of said salt, at a temperature between fromabout 800 F. to 950 F., for a period of time sufficient to change theoxides on said metals into a porous and at least loosely adherentreadily removable condition.

8. In a process for removing oxides from metals, the steps comprisingimmersing such metals in a bath of fused salt consisting essentially ofsodium hydroxide as the preponderant ingredient, potassium hydroxide inan amount within the range from about 0.25% to about 5% by weight of thetotal weight of said salt and the balance sodium nitrate in an amountgreater than the amount of potassium hydroxide but not greater than theweight of sodium hydroxide for a period of time sufficient to change theoxides on said metals into a porous readily removable condition, andthereafter immersing said metals in a dilute inorganic acid for a periodof time sufficient to complete the removal of said oxides.

9. In a process for removing oxides from metals, the steps comprisingimmersing such metals in a bath of fused salt consisting essentially ofapproximately equal parts of sodium hydroxide and sodium nitrate,potassium hydroxide in a quantity between about 0.25% and about 5% byweight of the total weight of said salt for a period of time sufficientto change the oxides on said metals into a porous and at least looselyadherent readily removable condition, and thereafter immersing saidmetals in a dilute inorganic acid for a period of time not in excess ofabout five minutes sufiicient to complete the removal of said oxides.

10. In a process for removing oxides from metals, the steps comprisingimmersing such metals in a bath of fused salt consisting essentially ofsodium hydroxide as the preponderant ingredient, potassium hydroxide inan amount within the range from about 0.25 to about 5% by weight of thetotal weight of said salt and an oxidizing agent selected from the groupconsisting of peroxide and oxygen yielding salts of the alkali andalkaline earth metals, said agent being stable in the presence of molteninorganic alkali in a quantity at least one and one-half times thequantity of said potassium hydroxide but not in excess of the weight ofsodium hydroxide for a period of time sufficient to change the oxides onsaid metal at least into a porous readily removable condition, andthereafter immersing said metals in a dilute inorganic acid for a periodof time sufficient to complete the removal of said oxides.

References Cited in the file of this patent UNITED STATES PATENTS Re.22,887 Spence et a1. June 3, 1947 2,271,375 McKay Ian. 27, 19422,567,456 Webster Sept. 11, 1951

1. A COMPOSITE SALT FOR A METAL CLEANING SALT BATH PROCESS, CONSISTINGESSENTIALLY OF, ON A WEIGHT BASIS, SODIUM HYDROXIDE AND SODIUM NITRATEIN APPROXIMATELY EQUAL PARTS AND POTASSIUM HYDROXIDE IN AN AMOUNT NOT INEXCESS OF ABOUT 10% OF THE TOTAL WEIGHT OF SAID SALT.
 3. IN A PROCESSFOR REMOVING OXIDES FROM METALS, THE STEPS COMPRISING IMMERSING SUCHMETALS IN A BATH OF FUSED SALT CONSISTING ESSENTIALLY OF SODIUMHYDROXIDE AS THE PREPONDERANT INGREDIENT, POTASSIUM HYDROXIDE IN AQUANTITY WITHIN THE RANGE FROM ABOUT 0.25% TO ABOUT 10% BY WEIGHT OF THETOTAL DRY WEIGHT OF SAID SALT, AND AN OXIDIZING AGENT SELECTED FROM THEGROUP CONSISTING OF PEROXIDE AND OXYGEN YIELDING SALTS OF THE ALKALI ANDALKALINE EARTH METALS, SAID AGENT BEING STABLE IN THE PRESENCE OF STRONGALKALI AT A TEMPERATURE BETWEEN FROM ABOUT 650* F. TO 1050* F. FOR APERIOD OF TIME SUFFICIENT TO CHANGE THE OXIDES ON SAID METALS INTO APOROUS READILY REMOVABLE CONDITION, SAID OXIDIZING AGENT BEING PRESENTIN A QUANTITY OF AT LEAST ONE AND ONE-HALF TIMES THE WIEGHT OF SAIDPOTASSIUM HYDROXIDE BUT NOT IN EXCESS OF THE WEIGHT OF SODIUM HYDROXIDE.